1. Field of the Invention
The present invention relates to a method and system for transmitting at least one client signal within a server signal which has a predetermined transmission bit rate, with the transmission bit rate of the client signal and the transmission bit rate of the server signal being different.
2. Description of the Prior Art
Although modern digital transmission systems which, for example, use the synchronous digital hierarchy (SDH) or the plesiochronous digital hierarchy (PDH), support different client signals in terms of the respective transmission rate and the respective transmission protocol, specific transmission-rate and transmission-protocol-specific processing steps are required to transmit the respective client signals. To do this, it is necessary to modify or convert the transmission units in the digital transmission system for transmitting client signals which have different transmission rates.
In digital transmission systems which have been implemented so far, digital server signals having a variable bit rate are provided which are coupled to the bit rate of the client signal to be transmitted; the transmission signal from a customer in the transmission system. Furthermore, digital transmission signals have a constant bit rate, in particular the digital server signal, and have a frame structure having an overhead area and a payload area. The payload area is intended for transmitting client signals, and the overhead area contains control and monitoring information. The client signal thus occupies the entire signal area (payload area) of the server signal. If the bit rate in the client signal changes, the bit rate of the server signal is appropriately modified or adapted. If the server signal does not have the transmission rate required for the client signal to be transmitted, it is normally necessary to modify or to reconfigure individual transmission units in the digital transmission system to do this. Such methods and arrangements are referred to in specialist circles by the term “digital rapper”.
For high transmission rates, for example in optical transmission systems, the range of bit rates which can be transmitted in the server signal is highly restricted owing both to technical implementation problems and for cost reasons. If the client signal bit rate varies, then the bit rate or transmission rate of the server signal is matched to it, requiring replacement or adaptation of all those components which are sensitive to the bit rate within the optical transmission system; for example, 3-R regenerators and clock recovery devices.
Furthermore, a mapping scheme is known from ITU-T Recommendation G.707, which defines the mapping of 34 Mbit/s and 45 Mbit/s PPH signals onto an SDH-VC-3-signal. A digital server signal is provided for this purpose, which has a constant bit rate and onto whose payload area the client signal is mapped. The payload area in the server signal is filled more or less effectively depending on the client signal bit rate. Fixed stuffing positions, flexible stuffing positions and fixed client data positions are defined for the mapping scheme in the ITU-T Recommendation G.707, resulting in a specific mapping scheme for each client signal having a different bit rate. Although the described method envisages a constant bit rate for the server signal, a mapping scheme defined by an ITU-T-Standard is, however, required for each client signal in order to ensure interworking between the communications units and transmission units from different manufacturers. The interfaces between the client signal which has a variable bit rate and the server signal which has a fixed bit rate must, for this purpose, be matched to a mapping scheme.
An object to which the present invention is directed is to improve the transmission of client signals which have a variable transmission rate within a server signal which has a constant transmission rate.